The present invention relates to stents. More particularly, the present invention provides apparatus and methods for stenting that are indicated for use in tortuous anatomy and in vessels that undergo temporary deformation, and furthermore that may be tailored to an appropriate profile in-vivo.
Stents are commonly indicated for a variety of intravascular applications, including restoration and/or maintenance of patency within a patient""s vessel. They are also used to prevent restenosis of the blood vessel post-dilation, thereby ensuring adequate blood flow through the vessel. In certain applications, for example, in the carotid arteries, stents must further prevent release of embolic material from the walls of the vessel. Blood flow may carry such embolic material downstream into the vasculature of the patient""s brain, where the material may occlude flow and cause stroke or other permanent damage to the patient.
Conventional stents are formed of a cell or mesh structure having interstitial spaces that limit the ability of such stents to prevent release of emboli. Thus, stent grafts often are used in order to seal stenotic emboli against the vessel wall. A stent graft comprises a stent, which is at least partially covered with a biocompatible material that is impermeable to stenotic emboli. In addition to preventing release of emboli, stent grafts are indicated for bridging defective points within a vessel, such as aneurysms, ruptures, dissections, punctures, etc.
The graft covering material may comprise a biocompatible polymer, such as Polyethylene Terephthalate (PET? or xe2x80x9cDacronxe2x80x9d) or Polytetrafluoroethylene (PTFE or xe2x80x9cTeflonxe2x80x9d), or, alternatively, the material may be homologic, for example, an autologous or non-autologous vein. PETP-covered stent grafts typically are only able to expand in the single dimension in which the fabric has been tensioned. Thus, the dimension of the vessel to be treated must be determined in advance, and potential for in-vivo diameter adjustment of PETP-covered grafts is limited.
Stent grafts may be either balloon-expandable or self-expandable. Advantageously, balloon-expandable systems may be expanded to an optimal diameter in-vivo that corresponds to the internal profile of the vessel. However, as compared to self-expandable stents, balloon-expandable stents are fabricated from relatively rigid materials, such as stainless steel. Balloon-expandable stents and stent grafts are therefore not indicated for use in tortuous anatomy or in vessels that may be temporarily deformed, for example, through contact with neighboring muscles, through joint motion, or through pressure applied externally to the patient.
Conversely, self-expandable stents and stent grafts characteristically return in a resilient fashion to their unstressed deployed configurations after being compressed and are thus indicated for use in tortuous anatomy and in vessels that undergo temporary deformation. Fabrication materials for self-expandable stents include superelastic materials, such as nickel-titanium alloys (xe2x80x9cNITINOLxe2x80x9d), spring steel, and polymeric materials. Alternatively, the stents may be fabricated from elastic materials comprising resilient knit or wickered weave patterns.
A drawback of self-expandable stents is that they have deployed diameters that cannot be adjusted in-vivo. Since it is difficult to accurately determine the internal diameter of a vessel, self-expandable stents are commonly implanted with deployed diameters that are too large or too small for a given application. It the selected stent diameter is too large, the stent applies a permanent pressure against the vessel wall, which over time may cause the vessel to expand and adjust to the geometry of the stent. This is highly undesirable, as it alters the natural flow characteristics of the vessel with unpredictable results. Alternatively, if the deployed diameter is too small, the stent may not tightly abut against the vessel wall. Turbulent flow may develop in the gap between the vessel wall and the stent, thereby leading to dangerous thrombus formation, or the stent may dislodge and flow downstream with potentially fatal consequences. Further still, the diameter of a vessel may change along its length, in which case selection of a properly dimensioned self-expandable stent is essentially not possible.
When used in a stent graft, self-expandable stents are typically covered with a biocompatible material that is dimensioned to correspond to either the expanded deployed, or the collapsed delivery configuration of the stent. When dimensioned for the deployed configuration, the stent is collapsed to the delivery configuration, and the biocompatible material is folded onto and bonded to the stent such that the material becomes taut only when the stent dynamically expands to the deployed configuration. When dimensioned for the delivery configuration, the material has sufficient elasticity to expand with the stent without limiting or preventing self-expansion of the stent. In either case, the stent dynamically expands to its fully deployed configuration, providing a medical practitioner with no opportunity to tailor the stent in-vivo to the patient""s unique anatomy.
In view of the drawbacks associated with previously known stents and stent grafts, it would be desirable to provide apparatus and methods for stenting that overcome these drawbacks.
It also would be desirable to provide apparatus and methods for stenting that allow in-vivo tailoring of stent diameter.
It further would be desirable to provide apparatus and methods for stenting that are indicated for use in tortuous anatomy and in vessels that undergo temporary deformation.
It would be desirable to provide apparatus and methods for stenting that are indicated for use at a vessel branching.
In view of the foregoing, it is an object of this invention to provide apparatus and methods for stenting that overcome the drawbacks of previously known apparatus and methods.
It is another object of this invention to provide apparatus and methods that allow in-vivo tailoring of stent diameter.
It is yet another object of the present invention to provide apparatus and methods for stenting that are indicated for use in tortuous anatomy and in vessels that undergo temporary deformation.
It is an object of the present invention to provide apparatus and methods for stenting that are indicated for use at a vessel branching.
These and other objects of the present invention are accomplished by providing apparatus for stenting comprising a self-expandable stent that is at least partially covered with a biocompatible material configured to prevent dynamic self-expansion of the stent. The biocompatible material is irreversibly expandable by suitable means, for example, by a balloon or other inflatable member, but has sufficient tensile strength and is attached to the stent in such a manner that hoop stress applied by the stent in the delivery configuration is not sufficient to achieve irreversible expansion of the material. Thus, the present invention provides apparatus that may be tailored in-vivo to a vessel profile, in a manner similar to a balloon-expandable stent or stent graft, but that maintains required flexibility for use in tortuous anatomy and in vessels that undergo temporary deformation, in a manner similar to a self-expandable stent or stent graft.
In a first embodiment, the biocompatible material preferably comprises a high-strength PTFE fabric or a homologic material that is wrapped around and tautly attached to the stent in a collapsed delivery configuration. The material is preferably impermeable to stenotic emboli. Additionally, the material may comprise a coating configured for localized delivery of therapeutic agents or for inhibition of thrombus formation.
The stent preferably comprises a superelastic material, such as a nickel titanium alloy, spring steel, or a polymeric material. Alternatively, the stent may be fabricated with a resilient knit or wickered weave pattern of elastic materials, such as stainless steel. At least a portion of the stent is preferably radiopaque to facilitate proper positioning of apparatus of the present invention within a vessel.
The apparatus is mounted on a balloon catheter in the delivery configuration for delivery to a treatment site. Upon delivery using well-known techniques, the balloon catheter is inflated with sufficient pressure to facilitate irreversible expansion of the biocompatible material and to anchor the apparatus against the vessel wall with an in-vivo tailored diameter. A plurality or balloons having different diameters may be used to further tailor the stent diameter to the profile of the vessel. Stent diameter may even be varied along the length of stenting by inserting a balloon only partially inside the stent during inflation, or by using balloons of lengths shorter than the length of the stent. Importantly, and in contrast to conventional balloon-expandable systems, embodiments of the present invention characteristically deform and return in a resilient fashion to their tailored configurations after being compressed or deformed by an outside force.
In an alternative embodiment, apparatus is provided for use at a vessel branching, wherein the stent and biocompatible material comprise a radial opening. When stenting at the vessel branching, the opening may be positioned in line with the side branch to maintain patency of the branch. Furthermore, a plurality of radial openings may be provided along the length of the implant as required to ensure continuous blood flow through a plurality of side branches.
Methods of using the apparatus of the present invention are also provided.